The present invention pertains generally to contact structures affixed to semiconductor lasers, and pertains more particularly to improved contact structures and methods of their manufacture that are inexpensive.
The efficient operation of semiconductor laser devices, especially laser devices that operate at high power levels, require electrical contacts that have several properties. The first property is low electrical resistance, which improves efficiency by minimizing resistive power losses in the electrical contacts and which improves operational characteristics of the laser by minimizing the amount of heating caused by these resistive power losses. The second property is a coefficient of thermal expansion (CTE) that closely matches the CTE of the semiconductor laser material, which improves reliability by reducing mechanical strain in the laser and in the bond between laser and contact caused by differing amounts of thermal expansion. Problems caused by CTE mismatch are especially troublesome for semiconductor laser bars having one or more dimensions on the order of a centimeter (cm).
In applications that assemble multiple P-N junction semiconductor lasers into a single laser module, it is often desirable to make both the P-side and the N-side electrical contacts as thin as possible so that the lasers can be stacked next to each other as closely as possible. The degree to which the thickness of these contacts can be reduced is restricted by the cooling requirements of the laser module. In many implementations, the heat generated by a P-N junction laser is confined mostly to the P-side of the laser. As a result, the P-side electrical contact should also have high thermal conductivity. This particular property is not as important to the N-side electrical contact in these implementations; therefore, another property for at least an N-side contact is that it can be made very thin. Of course, in implementations that have other heat-generating characteristics, thermal conductivity requirements imposed on the P-side and N-side contacts will differ.
Yet another property for an electrical contact is that it can be manufactured very inexpensively. This requires that the contact be made of materials that are themselves inexpensive to acquire, and that the manufacturing process used to make the contacts be inexpensive to carry out.
Unfortunately, no known electrical contact structure satisfies all of these properties simultaneously.
It is an object of the present invention to provide for an electrical contact structure that can closely match the coefficient of thermal expansion of a semiconductor laser, can have very low electrical resistance, can be made very thin, and can be made very inexpensively having dimensions with low tolerances.
According to one aspect of the present invention, a laser module comprises a PN-junction semiconductor laser having a coefficient of thermal expansion (CTE) and having a P side and an N side. The laser module also includes a contact structure affixed to a surface of the semiconductor laser, where the contact structure comprises a first component comprised of a first material having a first coefficient of thermal expansion (CTE); and a second component comprised of a second material having a second CTE plated on opposing surfaces of the first component. The first and second components have an overall thickness such that the contact structure has an effective CTE that matches the CTE of the semiconductor laser more closely than either the first CTE or the second CTE. The layers of the second material are of substantially equal thickness to balance thermal stresses between the first component and the second component